Several different types of devices are known wherein transmittance to electromagnetic radiation can be reversibly varied. Among such devices are those wherein the transmittance is changed by thermochromic, photochromic, or electro-optic (e.g., liquid crystal, dipolar suspension, electrophoretic, electrochromic) means and wherein the variable transmittance is to electromagnetic radiation that is at least partly in the visible range (wavelength from 4200 .ANG. to 7000 .ANG.).
Devices of reversibly variable transmittance to electromagnetic radiation have found application as the variable transmittance element in variable transmittance light-filters, variable reflectance mirrors, and display devices which employ such light-filters or mirrors in conveying information. These variable transmittance light filters have included windows. The variable reflectance mirrors have included anti-glare rearview mirrors for automotive vehicles.
Devices of reversibly variable transmittance to electromagnetic radiation, wherein the transmittance is altered by electrochromic means, including electrochemichromic devices, are descibed, for example, by Chang, "Electrochromic and Electrochemichromic Materials and Phenomena," in Non-emissive Electrooptic Displays, A. Kmetz and K. von Willisen, eds. Pergamon Press, New York, N.Y. 1976, pp. 155-196 (1976). Electrochemichromic devices includes those wherein electrochemical reactions occur in a solid film, involve electroplating or occur entirely in solution. See Chang, supra.
Numerous electrochemichromic devices are known in the art. See, e.g., Manos, U.S. Pat. No. 3,451,741; Bredfeldt et al., U.S. Pat. No. 4,090,782; Shattuck and Sincerbox, U.S. Pat. No. 4,093,358; Clecak et al., U.S. Pat. No. 4,139,276; Kissa et al., U.S. Pat. No. 3,453,038; Rogers, U.S. Pat. Nos. 3,652,149, 3,774,988 and 3,873,185; and Jones et al., U.S. Pat. Nos. 3,282,157, 3,282,158, 3,282,160 and 3,283,656. Among these devices are single-compartment, self-erasing, solution-phase electrochromic devices. See, e.g., Manos, supra, which is incorporated herein by reference; Bredfeldt et al., supra; Shattuck and Sincerbox, supra; and Clecak et al., supra.
In a single-compartment, self-erasing, solution-phase electrochromic device, the intensity of electromagnetic radiation is modulated by passing through a solution held in the device in a compartment which includes two electrodes. The two electrodes are in contact with the solution. Between the electrodes, there is no barrier, such as a semi-permeable membrane, which would divide the solution compartment and prevent some components in the solution from diffusing or migrating from one electrode to the other. The solution includes a solvent and at least one "anodic" compound (which can be neutral or charged) and at least one "cathodic" compound (which also can be neutral or charged). The "anodic" compounds are electrochemically oxidized and the "cathodic" compounds are electrochemically reduced when a DC electrical potential difference is impressed across the solution between the electrodes. If none of the "anodic" compounds and "cathodic" compounds to be oxidized or reduced is charged, prior to oxidation or reduction, respectively, the solution will, and otherwise the solution may, include inert, current-carrying electrolyte. The electrochemical properties of the solvent, inert, current-carrying electrolyte, if any, anodic compounds, cathodic compounds, and any other components that might be present in the solution are preferably such that the anodic and cathodic compounds are oxidized and reduced, respectively, at a potential difference between the electrodes which does not cause any significant electrochemical or other changes in the other components in the solution. The solution is fluid during operation of the device, although it may be gelled or made highly viscuous with a thickening agent. That the devices are "solution-phase" means that all of the components in the solution, including the anodic and cathodic compounds, remain in solution during operation of the device with the concomitant oxidation of anodic compounds and reduction of cathodic compounds.
Reversible modulation of intensity of electromagnetic radiation passing through a single-compartment, self-erasing, solution-phase electrochromic device can be accomplished because of three factors related to operation of the device. First, the molar extinction coefficients of the anodic compounds and cathodic compounds in the solution of the device, as a function of wavelength, change with their electrochemical oxidation and reduction, respectively. Generally, at least one of these compounds undergoes a significant change in extinction coefficient at wavelengths in the visible range upon the oxidation or reduction; consequently, the solution and device change color or change from dark to clear or clear to dark when a potential difference is applied across the solution between the electrodes. Second, in the solution, the oxidized anodic compounds and reduced cathodic compounds do not, to any significant extent, undergo degradative reactions unimolecularly or with other components. Third, in the solution, the oxidized anodic compounds react substantially only with the reduced cathodic compounds to yield substantially only anodic compounds and cathodic compounds in their forms and with their properties prior to the oxidations and reductions, respectively. These reactions of oxidized anodic compounds with reduced cathodic compounds provide the "self-erasing" feature to the device.
Heretofore, no single-compartment, self-erasing, solution-phase electrochromic devices have been known which have proven to be suitable for commercial application as the component of reversibly variable transmittance in variable transmittance light filters or variable reflectance mirrors. For such applications, the solution of variable transmittance must be highly stable to cycling, at least several thousands of times, from zero potential difference between the electrodes to a potential difference between the electrodes that is sufficient to cause significant change in transmittance and then back to zero again. In a typical device, the solution is held in a layer between planar, parallel, spaced-apart, transparent walls, on the inside surfaces of which (in contact with the solution) are coated thin layers of transparent, electrically conductive material which serve as electrodes and through which passes electromagnetic radiation whose intensity is reversibly modulated in the device. It is advantageous to have the solution layer as thin as possible, in order to minimize distortion of light passing through, or passing into and reflecting out of, a device, and to reduce to durations that are acceptable for commercial applications the "response time" required for the transmittance of a device to achieve a new steady-state value when the potential difference between the electrodes is changed. However, for devices with thin solution layers, anodic and cathodic electrochromic compounds must be found that, at concentrations in the solution at which they remain soluble, both at zero-potential equilibrium and when oxidized (in the case of anodic compounds) and reduced (in the case of cathodic compounds) when a potential difference is applied between the electrodes, give rise to sufficiently large changes in absorbance between their zero-potential equilibrium states and their "activated" (i.e., oxidized or reduced) states and at the same time remain sufficiently stable to cycling to provide a commercially practicable device. The present invention addresses the need for solutions to make commercially practicable single-compartment, self-erasing, solution-phase electrochromic devices.
A useful feature in such devices, that has not heretofore been available, is the capability to function as a gray-scale device, i.e., to vary continuously and rapidly in transmittance to light in the visible wavelength range as a function of the potential difference applied between the electrodes of the device. Such a "gray-scale" device would find application in a window, which would allow light of constant intensity to pass through independently of the intensity of the light reaching the window, and an anti-glare rearview mirror in an automobile, that would reflect light of acceptable intensity to the driver regardless of the intensity of the glare-causing light incident on the mirror from headlamps of automobiles approaching the vehicle from behind. The present invention provides gray-scaling capability in single-compartment, self-erasing, solution-phase electrochromic devices.
A problem that has not heretofore been recognized with solution-phase electrochromic devices is segregation, due to both migration and natural convection of anodic and cathodic electrochromic compounds. Particularly in devices that are operated continuously for long periods (more than about 20 minutes) with the planar surface through which light enters the device oriented vertically to the ground, such segragation can cause annoying and troublesome separation of color and reduction in speed of self-erasing. The present invention addresses this segregation problem.
Variable reflectance mirrors include a variable transmittance component, which is a device which has a transmittance to visible light which is reversibly varied by thermochromic, photochromic, or electro-optic means, and a reflection means, which is a highly reflective surface (such as a silver layer) from which light is reflected after passing through a medium of reversibly variable transmittance in the variable transmittance component. After reflecting from the reflection means, the reflected light passes back through the medium of reversibly variable transmittance. The medium of variable transmittance in such mirrors is typically held, in the variable transmittance component, between two planar, parallel, spaced-apart surfaces. At least one of these surfaces is transparent to light, and light reflected by the mirror enters and leaves through this transparent surface. A problem with such mirrors is the high "residual" reflectivity, which is usually greater than 5%, of this transparent surface of the variable transmittance component. For example, in an anti-glare rearview mirror for an automobile, wherein elimination of high glare may require reduction of reflectivity observed by the driver from all surfaces to as low as about 5 to 7%, the high residual reflectivity of the front surface of a typical mirror requires that the transmittance of the medium of reversibly variable transmittance in the mirror be capable of being made as low as about 3%. Because it is difficult to achieve such low transmittance with sufficient speed in preferably thin devices of reversibly variable transmittance, it would be advantageous to have variable reflectance mirrors wherein these problems caused by high residual reflectivity are avoided. The present invention provides such mirrors.